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Australia Research and Development Tax Incentive - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Research and Development Tax Incentive [Line Items]          
Unrealized foreign currency transaction gains (losses)     $ (74,000) $ 21,000  
Research and Development [Member]          
Research and Development Tax Incentive [Line Items]          
Percentage of reduction in research and development costs 43.50%        
Zafgen Australia Pty Limited [Member]          
Research and Development Tax Incentive [Line Items]          
Unrealized foreign currency transaction gains (losses) $ (100,000) $ 0 (100,000)    
Zafgen Australia Pty Limited [Member] | Australia [Member]          
Research and Development Tax Incentive [Line Items]          
Tax incentive receivable $ 1,800,000   1,800,000   $ 900,000
Zafgen Australia Pty Limited [Member] | Maximum [Member]          
Research and Development Tax Incentive [Line Items]          
Unrealized foreign currency transaction gains (losses)       100,000  
Zafgen Australia Pty Limited [Member] | Research and Development [Member]          
Research and Development Tax Incentive [Line Items]          
Percentage of refundable tax incentive 43.50%        
Reduction to research and development expenses $ 400,000 $ 200,000 $ 1,000,000 $ 200,000